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Genetics/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby An image shows a white mother cat nursing four kittens, which are brown, black, gray, and reddish brown. A robot, Moby, and a boy, Tim, are watching the animals. MOBY: Beep. TIM: I don't know. Maybe the father was a tabby. Tim reads from a typed letter. TIM: Dear Tim and Moby, Both my parents have brown eyes, and I have blue eyes. How can that happen? From, Emily P. TIM: Well, this is just a guess, but I'll bet one of your grandparents has blue eyes. Moby reaches two fingers over to one of Tim's eyes and opens the eye wide. TIM: Stop that.The color of your eyes is a factor of heredity, the passing down of physical traits from parents to children. An image shows a photograph of two parents, with two sons and a daughter. All of them have red hair and similar features. TIM: That includes things like hair type, eye color, bone structure, and even parts of your personality. These traits are carried in our genes. Images show a lock of hair, a blue eyeball, a skull, and a smiley face. TIM: Genes are made up of DNA, which stands for deoxyribonucleic acid. An image shows a strand of DNA. TIM: DNA is arranged in pairs of strands called chromosomes. An image shows two chromosomes. TIM: In humans, there are 23 pairs of chromosomes in each cell. That's 46 chromosomes in all. An image shows 23 pairs of chromosomes. The pairs are different sizes, but the strands of DNA within each pairlook the same. TIM: Just about every cell in your body has these 46 chromosomes in it. An image with a magnification shows a cell and its chromosomes. TIM: You can think of them as a computer program for your body. An animation shows a computer screen with code related to a DNA strand. TIM: Because everybody's different, no two sets of DNA are alike. An animation shows a computer monitor displaying DNA and chromosomes. TIM: You get your genes from your parents, 23 chromosomes from your mother and 23 chromosomes from your father. Your parents got their genes from their parents, and so on. An animation shows Tim's genealogy chart, which includes Tim, his parents, their parents, and so on. TIM: The genes from your parents combine to form your genes. An animation shows two sets of chromosomes combining into one set. MOBY: Beep. TIM: Organisms that reproduce sexually, like people, have special cells just for making offspring. In animals, male sex cells are called sperm and female sex cells are eggs. An animation shows a sperm cell swimming toward an egg. TIM: Sex cells are special because they have only a half set of genes. An image shows two separate half-sets of chromosomes. TIM: If your mother has brown eyes, and your father has blue eyes, the chances are your eyes will be brown. Images show the faces of Tim's parents. Each has a chromosome beneath it. Tim's father has the chromosome for blue eyes, and Tim's mother has the chromosome for brown eyes. Tim's face appears and shows that he has the chromosome for brown eyes. TIM: This is because the genes that code for blue eyes are recessive, and the genes that code for brown eyes are dominant. The chromosomes are labeled as Tim describes. TIM: That means that in most cases, the gene for brown eyes will block out the gene for blue eyes. You still have the blue-eyed genes in your DNA, but they're just not expressing themselves. An animation shows a blue eyeball next to the blue-eye chromosome. When the brown-eye chromosome appears, the eye becomes brown. TIM: By the same logic, two parents with brown eyes might be carrying recessive blue-eyed genes. If a sperm containing this recessive gene comes together with an egg containing the recessive gene, the brown-eyed parents can make a blue-eyed baby. An animation shows a genealogy chart with eye-color chromosomes as Tim describes. Genes from two brown-eyed chromosomes combine to form a blue-eyed chromosome. MOBY: Beep. TIM: Really? MOBY: Beep. TIM: Huh. Well, I guess you're right. Moby says that the genetic process that determines your eye color is a little bit more complicated than this. It actually involves several genes, not just one. But it's a good example of how traits are passed down from generation to generation. An image shows a genealogy chart. TIM: Of course, it's important to remember that genes don't determine everything. What we look like is largely determined by the genes we inherit, but factors like environment and nutrition can play a role, too. For example, modern humans are taller on average than our ancestors because of better nutrition. But it's hard to overestimate how much our genes shape who we are. MOBY: Beep. TIM: Are my parents nerds? Well, my dad, um, why do you ask? Category:BrainPOP Transcripts